Many commercially available electrochemical sensors are of the amperometric type, in which two or more electrically conductive electrodes are connected to an external circuit and communicate through an ion conductive material, called an electrolyte. When a detectable species is present, a current proportional to the species concentration is produced in the external circuit due to a REDOX reaction at the electrode surfaces. The electrodes in these sensors typically include a powder-based conductive material, which may include a catalyst such as platinum, supported by a gas permeable membrane, such as a TEFLON® film. Two or more such electrodes are then assembled in a 2D or 3D configuration within a housing and are separated from each other by an electrolyte. The electrolyte may be a liquid based electrolyte, such as an aqueous salt or acid, a gel, or solid based electrolyte, such as YSZ ceramic or a solid polymer electrolyte.
In operation, the electroactive species to be detected diffuses through the gas permeable membrane and makes contact with a first electrode, called the working electrode, where it undergoes an oxidation or reduction half reaction. Examples of half reactions on a working electrode include the following.Carbon Monoxide Oxidation: CO+H2O→CO2+2H++2e−Hydrogen Sulfide Oxidation: H2S+4H2O→H2SO4+8H++8e−Nitrogen Dioxide Reduction: NO2+2H++2e−→NO+H2OChlorine Reduction Cl2+2H++2e−→2HClHydrazine Oxidation: N2H4+4OH−→N2+4H2O+4e−Produced ion products are then transported across the electrolyte to a second electrode, called the counter electrode, where a counter oxidation or reduction half reaction occurs. The counter oxidation reactions may be accomplished, e.g., by the reduction of protons with oxygen to form water. The counter reduction reactions may be accomplished, e.g., by breaking water into hydrogen ions and oxygen. A third reference electrode optionally is utilized to improve stability of the electrical output, by maintaining a fixed potential via an external circuit.
The fine catalyst powders distributed on the membrane to produce the electrodes described above can result in high electrical resistance in the electrodes due to the discontinuous nature of the powder. In addition, the often used permeable membranes, such as those made of fluoropolymers, are hydrophobic in nature, making it difficult to maintain intimate contact with the electrolyte. Both of these factors can contribute to a higher overall cell resistance, which can result in lower current output. Electrical output from the electrodes is typically accomplished by pins which protrude out of the cell body. For example, the electrodes may be connected to the pins by a current carrying wire or ribbon sandwiched into the assembly. The net result of this type of construction can be higher costs, lower part-to-part repeatability, and lower durability.
Examples of such sensors are described in U.S. Pat. Nos. 4,227,984, 5,126,035, 5,304,293, 5,338,429, 5,650,054, 5,906,718, 5,914,019, 6,200,443, 6,454,923, 6,908,357, 6,948,352, and in U.S. Published Patent Application No. 2002/0029613.